1. Field of the Invention
This invention relates generally to triple-mode cavity filters for microwave and radio frequency signals and, more particularly, to cavity filters using metallic resonators.
2. Description of the Related Art
Wireless communication systems often require devices to select signals within predetermined frequency bands. When these devices are implemented as bandpass filters, users can select a desired range of frequencies, known as a passband, and discard signals from frequency ranges that are either higher or lower than the desired range. It is particularly important for bandpass filters to achieve high out-of-band rejection, attenuating signals outside the passband to emphasize the desired frequency range.
Cavity resonators are devices frequently used to implement bandpass filters. A cavity resonator confines electromagnetic radiation within a solid structure, typically formed as a rectangular parallelepiped. Other cavity shapes maybe used, such as cylinders and spheres. Because the enclosed cavity acts as a waveguide, the pattern of electromagnetic waves is limited to those waves that can fit within the walls of the waveguide. Within the cavity, the reflection of the waves can result in a variety of patterns, known as resonant modes.
In order to reduce the cost and the size, it is often necessary to replace multiple cavity resonators with a single cavity resonator. A single physical cavity can function in the same manner as two cavities if two of its resonant modes are set to the same resonant frequency, making it a dual-mode resonator. The design can be further improved by using three degenerate resonant modes. In this configuration, known as a triple-mode resonator, three resonant modes of the resonator, resonating near each other, are used to construct a filter function. In other words, one cavity accommodates three electromagnetic resonances that are employed in the construction of the filter response.
One structural design of a triple-mode resonator structure uses a dielectric cube as a resonator. While this structure produces three modes that resonate at similar frequencies, the dielectric cube resonator has a number of disadvantages. Fabrication of dielectric resonators with expensive ceramic materials would make the overall filter more costly.
The dielectric cube also tends to produce spurious resonances near the resonator's desired operating frequency. Aggressive suppression is needed to discard these unwanted frequencies. While suppression would compensate for the spurious modes, it would also greatly increase the insertion loss of the resonator. An increase in insertion loss is proportional to a decrease in transmitted power from the resonator. Therefore, the elimination of spurious modes also reduces the overall signal strength.
Accordingly, there is a need to produce a triple-mode resonator that overcomes the detrimental characteristics of the dielectric cube structure. More particularly, there is a need for a triple-mode resonator that is relatively inexpensive to manufacture and has a wide, spurious-free response.
The foregoing objects and advantages of the invention are illustrative of those that can be achieved by the various exemplary embodiments and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the various exemplary embodiments will be apparent from the description herein or can be learned from practicing the various exemplary embodiments, both as embodied herein or as modified in view of any variation which may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel methods, arrangements, combinations, and improvements herein shown and described in various exemplary embodiments.